Whether in lithographic or other processes, it is desirable to make inspections or measurements using an optical detection measuring technique. Further, it is desirable to use radiation in other processes such irradiation of surfaces, in telecommunications, etc.
To at least facilitate transport of the radiation, optical fiber may be used. An optical fiber is a waveguide (typically cylindrical) that transmits radiation along its axis. The fiber typically comprises a core (usually in the middle) surrounded by a “cladding” layer. The fiber may be made entirely from solid transparent materials such as glass; the core and “cladding” layer are typically made of dielectric material. The transparent material in one part (usually the middle) of the cross-section of the fiber has a different optical structure, e.g., higher refractive index, than the rest and forms the core within which the radiation is guided by, e.g., total internal reflection. The boundary between the core and “cladding” layer may be abrupt, in, e.g., step-index fiber, or gradual, in, e.g., graded-index fiber. The optical fiber may be single mode or multi-mode, wherein a main difference between multi-mode and single-mode optical fiber is that the former has a significantly larger core cross-wise dimension (e.g., width or diameter) e.g., typically 50-100 micrometers, while the latter typically has a core cross-wise dimension less than about ten times the wavelength of the propagating radiation, e.g., 8 and 10.5 μm.
A photonic-crystal fiber (PCF) is a special form of optical fiber. PCF can come in variety of forms and are based on the properties of photonic crystals (although the fiber itself need not have a crystal material). An example of PCF comprises a single solid, and substantially transparent, material, such as fused silica glass, within which is embedded a periodic array of open holes, running parallel to the fiber axis. A “defect” in the form of a single missing hole within the regular array forms a region of raised refractive index, which acts as a waveguiding fiber core within which radiation is guided, in a manner analogous to total-internal-reflection guiding in standard fibers. Another mechanism for guiding radiation is based on photonic-band-gap effects. Photonic-band-gap guidance can be obtained by suitable design of the array of holes.